FIG. 1 shows a retransmission protocol 100 for current WCDMA systems (e.g., WCDMA Release 5/6) in which high speed data transmission can be achieved by means of high speed downlink/uplink packet access (HSDPA/HSUPA) technology. To improve the reliability of data transmission, multiple retransmission loops are enabled therein.
The protocol 100 utilizes a user equipment (UE) 102, a Node B 104, and a radio network controller (RNC) 106. The UE 102 includes a physical (PHY) layer 110, a medium access control (MAC) layer 112, and a radio link control (RLC) layer 114. The Node B 104 includes a PHY layer 120 and a MAC layer 122. The RNC 106 includes a MAC layer 130 and an RLC layer 132.
First, a HARQ process 140 runs in the MAC layers 112, 122 between the UE 102 and the Node B 104. The packets are assigned sequential transmission sequence numbers (TSNs) in a HARQ entity at the transmitter. At the receiver, a corresponding HARQ entity receives the packet transmissions and attempts to decode and recover each transmitted packet. The receiver HARQ entity is tasked with providing the recovered packets to higher layers in the proper order. Since individual packets may require a different number of retransmissions for successful delivery, a reordering entity is used at the receiver to buffer and reorder the packets.
When a packet is not decoded correctly, the receiver sends a negative acknowledgment (NACK) to the transmitter to initiate a retransmission of the packet. Otherwise, the receiver sends an acknowledgment (ACK) for the corresponding packet. Upon receiving a NACK, the transmitter retransmits the corresponding packet if the number of allowed retransmissions for the HARQ packet is less than a predetermined maximum value. Otherwise, the packet is discarded by the HARQ process, and retransmission of the packet is handled by an ARQ process 142 in the RLC layers 114, 132 between the UE 102 and the RNC 106.
The packet may not be correctly received in the RLC layer of the receiver side (i.e., the RLC layer of the UE 102 or the RNC 106) due to a decoding failure in the HARQ process or packet losses between the Node B 104 and the RNC 106. To compensate for these situations, an ARQ process 142 is provided in the RLC layers 114, 132 between the UE 102 and the RNC 106. The packets are assigned sequence numbers (SNs) in the RLC entity at the transmitter similar to the TSNs and the packets are held in a retransmission buffer until the transmitter receives an ACK for the packets from the receiver.
The RLC in the receiver generates status protocol data units (PDUs) containing an ACK or a NACK and sends the PDUs to the transmitter, so that the RLC in the transmitter can retransmit the missing packet or delete the correctly transmitted packet from the retransmit buffer. Some of the packets may not be received correctly after several retransmission attempts in the RLC layer or before a packet discard timer for the packets has expired. The RLC in the transmitter then discards the packets and informs the upper layer and the receiver. The receiver RLC layer reorders the received packets and initiates a procedure for a missing packet.